Peripheral Nervous Systems : Spinal Nerves & Plexuses

As an Endurance Athlete we all suffer ongoing injuries, I have suffered so many that have been cause by overuse and imbalance of my muscles. The problem that I have faced many times, is nerve entrapment and damage resulting from these overuse issues. Here is my best shot at the nerves in their most basic layout - hope it helps.

We have 31 pairs of spinal nerves and they are named after the sec­tion of the spine they come out of.

• 8 pairs of cer­vi­cal nerves (C1-C8)
• 12 pairs of tho­racic nerves (T1 – T12)
• 5 pairs of lum­bar nerves (L1-L5)
• 5 pairs of sacral nerves (S1-S5)
• 1 pair of coc­cygeal nerves (Co1)

Why do we have an 8^th pair of cer­vi­cal nerves if we only have 7 cer­vi­cal ver­te­brae you ask? Well because we start at the top of the atlas and all the nerves are in between the ver­te­brae so the 8^th pair is in between C7 and T1.


Note the dif­fer­ence between roots and rami (ramus is sin­gu­lar). The rootlets extend out of the spinal cord medi­ally and com­bine to form roots. The roots com­bine to form the spinal nerve and then they split apart again and are now called rami (ramus for sin­gu­lar). The dor­sal root is pos­te­rior to the ven­tral (front) root.
The dor­sal rami inner­vate the deep mus­cles of the back for motor con­trol, such as the erec­tor spinae and also a hor­i­zon­tal strip of skin for sen­sory input.
The ven­tral rami in the tho­racic region are known as the inter­costal nerves. They run deep to the ribs and inner­vate the inter­costal mus­cles and pro­vide sen­sory input for the over­ly­ing hor­i­zon­tal strips of skin there as well as the abdom­i­nal wall mus­cles (motor) and skin (sen­sory). The inner­va­tion of the back, ribs, and abdom­i­nal area is shown here:

Nerve Plexuses

A nerve plexus is a net­work of nerves that seem to be tan­gled that mostly serve the limbs. Each net­work is a net­work of nerves that come together and then redis­trib­ute them­selves out with a dif­fer­ent dis­tri­b­u­tion of nerves into the limbs. They are made from the rest of the ven­tral rami (that is, from other than the tho­racic region that we haven’t talked about yet). Each end of each plexus con­tains fibers from sev­eral spinal nerves. The fibers from each ven­tral ramus trav­els along dif­fer­ent routes so that each limb mus­cle receives inner­va­tion from more than 1 spinal nerve to have a backup plan in case of injury.

We have four plexuses: Cer­vi­cal, Brachial, Lum­bar, and Sacral

Cer­vi­cal Plexus

This plexus is located under­neath the ster­n­oclei­do­mas­toid mus­cle (from C1 – C4). Most of the branches inner­vate the skin of neck and deep neck mus­cles. The Phrenic nerve (C3, C4, C5) gets spe­cial men­tion that inner­vates the top of the diaphragm (after trav­el­ing down through tho­racic cav­ity, along either side of the heart). Note that if both phrenic nerves are cut or if the spinal cord is sev­ered above C3, breath­ing stops.

The Brachial Plexus

This plexus gets con­tri­bu­tion from the tran­si­tional area from neck into armpit (axilla) between C5 and T1. It pro­vides almost all the inner­va­tion of the upper limb (this includes back and scapu­lar mus­cles that con­trol the limb). The orga­ni­za­tion of this plexus is also very messy. There’s mix­ing of branches and then a branch­ing of branches and then mix­ing again. Note how there’s divi­sions, trunks and roots.

Five nerves inner­vate the mus­cles of the arm:

1. Mus­cu­lo­cu­ta­neous inner­vates the biceps brachii and some over­ly­ing skin. (C5, C6, C7)
2. Median inner­vates ante­rior fore­arm mus­cles, some intrin­sic hand mus­cles, and skin of lat­eral (anatom­i­cal position/toward thumb) palm. (C5, C6, C7, C8, T1)
3. Ulnar inner­vates intrin­sic hand mus­cles and skin of the medial hand. (C8, T1)
4. Radial trav­els along radius and inner­vates Tri­ceps brachii and mus­cles of the pos­te­rior com­part­ment, and over­ly­ing skin. (C5, C6, C7, C8, T1)
5. Axil­lary inner­vates the del­toid and teres minor, and over­ly­ing skin. (C5, C6)

The Lum­bar Plexus

This plexus lies within the iliop­soas mus­cle and comes from between L1 and L4. The main branches inner­vate the ante­rior thigh. Smaller branches inner­vate some of the abdom­i­nal wall and iliop­soas muscle.

Femoral nerve — inner­vates quadri­ceps femoris mus­cle and over­ly­ing skin

Obtu­ra­tor nerve — inner­vates adduc­tor mus­cles and over­ly­ing skin

The Sacral Plexus

The sacral plexus lies cau­dal to the lum­bar plexus (stems from L4 to S4) and is often referred together with­the lum­bar plexus as the lum­bosacral plexus. The branches inner­vate but­tocks, pelvis, per­ineum and lower limb (except for ante­rior and medial thigh).

Sci­atic nerve, the largest nerve of the sacral plexus is actu­ally two nerves wrapped in one sheath:

Tib­ial nerve (L4,L5,S1,S2,S3) inner­vates pos­te­rior thigh, pos­te­rior leg (lower leg), and intrin­sic mus­cles of foot

Com­mon fibu­lar (per­oneal) nerve (L4,L5,S1,S2) inner­vates mus­cles of the antero­lat­eral leg (lower leg)

Carbo Loading before the big Race

Aside from tapering, carbo-loading is also one of those practices that a lot of runners think about but are never quite sure how to do properly.  For something as straightforward as ingesting more carbohydrates prior to running a marathon, it's actually not that easy to get it right. 

Also, some people swear by it and others...well not so much.

BUT if you are thinking about carbo-loading for your next race, here's a handy guide for you.  I hope I was able to simplify it down to a manageable level so as to encourage rather than discourage you from trying it.

The overall premise is that the glycogen stored in our bodies is good enough for 90 to 120 minutes of activity;  after that, you're on your own.  Some reasearchers claim that replenishing your carbs while racing is not all that effective because it takes time for your body to process it to a usable state.

Mayo Clinic writes that carbo-loading of 4g/kg of body weight is sufficient but a write-up by Alex Hutchinson (Sweat Science, Runner's World) in the Globe and Mail citing a London study says that up to 10g/kg is closer to the correct amount but most people are only able to consume ~7g/kg.  10g/kg is too much food for most people!

Runners who successfully loaded prior to racing ran 13.4% faster and slowed down less than the non-loaders in the final 7 kilometers. 

Another factoid to consider is that different types of carbs are absorbed in different ways.  For example, the carbs in pasta are absorbed via different channels in the body as, say, a sports drink.  In addition, the carbs in pasta are more complex and takes more time and calories to break down.  Ingesting different types of carbs at the same time leads to more efficient loading than eating or drinking more of just one type of carbs.

Lastly, there's a debate on what's more effective:  fueling days before the race or on the day before?  Some folks advice slowly stepping up your consumption 2-3 days before the race and then eating a normal meal the day before to prevent GI distress while others say load up on the day before.

Don't be worried if you gain a few pounds while loading...this is because 1 g of carbs absorbs 2-3 g of water.  You'll lose this water weight quickly enough as you're running your race.

So how much is enough?

Please note that I am not a nutritionist and the meals listed above are not recommendations but are simply illustrations of how much carbs are needed to reach the levels indicated.  It's a lot of food!   

As you can see, eating 10g/kg of bodyweight is not easy, unrealistic even for most people.  Alex Hutchinson writes that one way that might help is to top up your carbs on the day of the race.  This is accomplished by waking up a few hours before the race and consuming a sports drink to restore the glycogen that was used up while asleep (needed to maintain normal brain and body function).

The experiment continues and I will attempt to load up for 2 days and then eat normally on the day before the race.  Let's hope it's enough to take me past the finish line without hitting the wall again (oh how I hate that wall).